Multiple Sclerosis was once thought to be caused by a break down of oligodendocytes, which create the myelin sheathing that surrounds nerves. This myelin acts as an insulator on our nerve cells, called neurons, that help to transmit nervous impulses at a faster, more efficient rate. And this break down in turn created a break down in the myelin from an autoimmune response that broke down the myelin. Essentially, the body was attacking its own myelin.
However, that hypothesis has been refuted through experiments conducted on mice. Researchers, through genetic manipulation, damaged oligodendrocytes and waited for the immune response; but the response never came. Therefore, it has been posited that the damaging of oligodendrocytes did not lead to an autoimmune response, a reaction, but that the autoimmune response was actually the cause independent of oligodendrocyte breakdown.
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